Descriptions of Instruments 11 



being placed in a liorizontal position l:)y setting the vernier zeros of the vertical circle at 

 and 180. The needle, 120 mm. long, is mounted in a brass case with index mirrors at the 

 north and south ends, and readings are effected by shifting the instrument until the needle 

 point and its reflection in the mirror are in the same vertical plane with the index line on 

 the mirror surface. A balancing weight for the needle is mounted on a long screw. The 

 needle may be balanced by removing the end cap provided with a brass knob after removal 

 of the three screws holding the cap in place; the glass cover-plate is then drawn out, the 

 needle taken from the tube, and the balance weight screwed in or out, as may be necessary. 

 In replacing the needle, it is first replaced on the pivot, and the cover glass and cap are then 

 moimted. 



The coil is rotated by means of a flexible shaft consisting of two closely wound spirals 

 of jihosphor-bronze wire operated by two gears attached to a crank handle. 



The galvanometer (Plate 3, Figs. 3 and 4) is of the Kelvin type. The spindle center for 

 this instrument is mounted in a hard-rubber base 100 mm. in diameter and 12 mm. thick, 

 which in turn is supported by three leveling foot-screws insulated from the center by the rub- 

 ber base. The four coils are each wound with 80 grams of double-wound silk magnet wire. 

 Brown and Sharpe gage No. 34; there are 1,880 turns in each coil. The outside diameter 

 of the winding of each coil is about 30 mm., the inside 4 mm., and the thickness about 

 9 mm. The resistance of each coil is 86 ohms. They are so arranged that they may be 

 easily connected either in series or multiple. The terminals are each lettered, and the 

 coils are in series when the connections are made in the order of the alphabet, namely, 

 A, B, C, D, E, F, G, and H. The connections from the earth inductor are made at A 

 and H. Each pair of coils is mounted on a hinged frame, so that the galvanometer house 

 may be easily opened and new suspension inserted or other repairs made, as may be 

 necessary. The spools of the coils are carefully insulated from the brass parts of the 

 instrument by the use of hard rubber. 



The magnet system, consisting of six magnets, is mounted in two groups of three by 

 means of sealing wax in two light brass frames supported by a phosphor-bronze wire. 

 In order that the system be astatic as closely as possible, the magnets are very carefully 

 made and magnetized in accordance with the methods followed in the manufacture of 

 magnetometer magnets. The six magnets used for the galvanometer system consist of 

 two 8 nmi. long and 0.8 mm. in diameter, and four 6.4 mm. long and 0.8 mm. in diameter. 

 The sheet-brass pieces which carry the magnets are 0.15 mm. thick, and each stamped 

 with grooves to support the magnets. The phosphor-bronze wire supporting the damping 

 vane, mirror, and magnet system is 0.6 mm. in diameter. The air-damping vane of sheet 

 brass is 0.05 mm. thick, 8 mm. wide, and 20 mm. long, and mounted at center of system. 

 The mirror is attached directly to the light brass damping vane by tongues stamped from 

 the vane itself. The air damping is effected by means of the brass vane and a rectangular 

 box provided between the coils; in the half of this box mounted on one coil frame there 

 has been mounted at the center a V-shaped piece to increase the efficiency of the air 

 damping. Attachments for control magnets are provided on the suspension tube and on a 

 support mounted below the hard-rubber bearing-plate. These arrangements are friction- 

 tight, so that the control magnets may be adjusted as desired either vertically or in azimuth. 

 The control magnets are made very small, and it has been found that in case all are used 

 no effect is produced on the inclination when the galvanometer is mounted at a distance 

 of one meter from the inductor. At Washington, D. C, it has not been found necessary 

 to make use of the control magnets when the coils are connected in series. The suspension 

 is of very fine cocoon fiber, about 0.006 mm. diameter. The fiber is attached to a rod 

 mounted in a ring, in which it may be clamped by means of a set-screw, and the height 

 so adjusted. This ring in turn rests upon a second ring permanently attached to the 

 suspension tube, on which there is a second clamping-screw, which serves to permit turning 



